1. Field of Invention
The present invention relates to a magnetic memory device, and more particularly to a technology of using a capping layer to improve the property of the magnetic memory device.
2. Related Art
Magnetic random access memory (MRAM) is a new memory device having the advantages of being non-volatile, having high density, high read/write speed, and anti-radiation property. When writing data, a common method employs two current lines, a bit line and a word line, to sense magnetic memory cells selected by the intersection of magnetic fields, so as to change the magnetoresistance (MR) value by changing the magnetization vector direction of a free layer (or referred as a sensing layer). When reading data from the memory, the selected magnetic memory cells are connected to the read current source, so that the digital value of the memory data can be determined from the read resistance value.
Such a magnetic memory cell has a multi-layered stacking structure of magnetic materials, which is formed by stacking an anti-ferromagnetic material layer, a fixing layer formed by ferromagnetic/non-magnetic metal/ferromagnetic materials, a tunneling barrier insulating layer, and a free layer made of ferromagnetic materials. The status of the memory “0” or “1” is determined by a parallel or anti-parallel structure of the magnetization vector of two layers of ferromagnetic materials. However, when the magnetic memory is designed towards the trend of having a high density, the size of the magnetic memory cell must be reduced, resulting in a greater switching field of the free layer, which not only requires a greater current, but also causes a great challenge for the circuit design. Therefore, in designing a magnetic memory device, besides reducing the current, another important task is to achieve the consistency in switching the free layer of all the magnetic memory cells.
Moreover, how to shorten the response time required by the magnetic memory device for processing data and raise the MR ratio of the magnetic device is also an important issue in developing magnetic memory devices. Thus, currently, the MR ratio can be raised in two aspects. One aspect lies in researching and developing the tunneling layer (for example, MgO) of the magnetic device, through which it is found that the MR ratio can be effectively raised to above 200%. Meanwhile, in order to take the magnetic switching property into consideration, the material of the free layer must be changed from CoFeB with a high MR ratio into NiFe magnetic materials with a low magnetostriction coefficient, so as to solve the consistency problem in switching the free layer. However, the MR ratio would be greatly decreased, which is great problem in the design of the magnetic device. The other aspect lies in development of AIOx tunneling layer device, in which different capping layers are employed to improve the thermal stability and the MR ratio.